1. Field of the Invention
The present invention relates to a cold cathode fluorescent lamp used as a light source for a liquid crystal display panel and a liquid crystal display device including a backlight using the cold cathode fluorescent lamp. In particular, the present invention relates to the structure of electrodes in the cold cathode fluorescent lamp.
2. Background Art
Among various illuminating devices, discharge tubes are often used as compact extended-lifetime light sources with low power consumption and high luminance. Fluorescent lamps are well-known as low-pressure discharge tubes each comprising a glass tube, whose inner surface is coated with a fluorescent material, filled with an inert gas and mercury. The low-pressure discharge tubes include two types, i.e., a hot cathode type for thermoelectron emission and a cold cathode type for electron emission.
Since liquid crystal display panels are non-self-luminous, each liquid crystal display panel has an external illuminating device. The liquid crystal display panel controls the amount of transmitted illumination light and that of reflected one to display an image. The external illuminating device is disposed on the back or front of the liquid crystal display panel. As for the current mainstream structure of display devices requiring high luminance, an illuminating device is arranged on the back of a liquid crystal display panel. The illuminating device on the back of the liquid crystal display panel is called a “backlight”.
Backlights are broadly divided into a side-edge type and a direct-lighting type. A side-edge backlight has a structure in which a linear light source is disposed along one side edge of a light guide plate comprising a transparent plate. Side-edge backlights are generally used in display devices required to be thin for use in personal computers. On the other hand, direct-lighting backlights are generally used in large liquid crystal display devices for use in display monitors and television receivers. Each direct-lighting backlight has a structure in which the backlight (illuminating device) is disposed on the back of a liquid crystal display panel.
For example, an illuminating device for a liquid crystal display device includes, as a light source, a cold cathode fluorescent lamp (CCFL) that emits electrons and excites a fluorescent material using the electrons to emit light. Electrodes for emitting electrons are typically made of a metallic material, such as nickel. Since such electrodes have the sputtering characteristics, the electrodes wear out during operation. When the gas pressure in a glass tube is reduced in order to obtain high luminance, sputtering is accelerated. The acceleration of sputtering makes the electrodes to be easily broken. Unfortunately, shortening of the lifetime of the cold cathode fluorescent lamp becomes pronounced.
To reduce the cost of a liquid crystal display device, it is desirable to reduce the number of cold cathode fluorescent lamps used. To reduce the number of cold cathode fluorescent lamps while the luminance of a backlight is maintained, each cold cathode fluorescent lamp has to have high luminance. However, when the gas pressure is reduced in order to obtain high luminance, the lifetime of each cold cathode fluorescent lamp is shortened.
To increase the lifetime of the cold cathode fluorescent lamp while high luminance is realized by reducing the gas pressure, it is desirable to use electrodes having a low sputtering rate. So long as the sputtering rate is low, even when fast particles collide with the electrodes, the rate of atom emission is low. Such a technique is disclosed in, for example, JP-A-2004-178875.